Low metal porous silica dielectric for integral circuit applications

ABSTRACT

The invention relates to the production of nanoporous silica dielectric films and to semiconductor devices and integrated circuits comprising these improved films. The nanoporous films of the invention are prepared using silicon containing pre-polymers and are prepared by a process that allows crosslinking at lowered gel temperatures by means of a metal-ion-free onium or nucleophile catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of nanoporous silicadielectric films and to semiconductor devices and integrated circuitscomprising these improved films. The nanoporous films of the inventionare prepared using silicon containing pre-polymers and are prepared by aprocess that allows crosslinking at lowered gel temperatures by means ofa metal-ion-free onium or nucleophile catalyst.

2. Description of the Related Art

As feature sizes in integrated circuits are reduced to below 0.15 μm andbelow, problems with interconnect RC delay, power consumption and signalcross-talk have become increasingly difficult to resolve. It is believedthat the integration of low dielectric constant materials for interleveldielectric (ILD) and intermetal dielectric (IMD) applications will helpto solve these problems. While there have been previous efforts to applylow dielectric constant materials to integrated circuits, there remainsa longstanding need in the art for further improvements in processingmethods and in the optimization of both the dielectric and mechanicalproperties of such materials used in the manufacture of integratedcircuits.

One type of material with a low dielectric constant is nanoporous silicafilms prepared from silicon containing pre-polymers by a spin-on sol-geltechnique. Air has a dielectric constant of 1, and when air isintroduced into a suitable silica material having a nanometer-scale porestructure, such films can be prepared with relatively low dielectricconstants (“k”). Nanoporous silica materials are attractive becausesimilar precursors, including organic-substituted silanes, such astetraacetoxysilane (TAS)/methyltriacetoxysilane (MTAS)-derived siliconpolymers are used as the base matrix and are used for the currentlyemployed spin-on-glasses (“S.O.G.”) and chemical vapor deposition(“CVD”) of silica SiO₂. Such materials have demonstrated high mechanicalstrength as indicated by modulus and stud pull data. Mechanicalproperties can be optimized by controlling the pore size distribution ofthe porous film. Nanoporous silica materials are attractive because itis possible to control the pore size, and hence the density, mechanicalstrength and dielectric constant of the resulting film material. Inaddition to a low k, nanoporous films offer other advantages includingthermal stability to 900° C.; substantially small pore size, i.e., atleast an order of magnitude smaller in scale than the microelectronicfeatures of the integrated circuit; preparation from materials such assilica and tetraethoxysilane (TEOS) that are widely used insemiconductors; the ability to “tune” the dielectric constant ofnanoporous silica over a wide range; and deposition of a nanoporous filmcan be achieved using tools similar to those employed for conventionalS.O.G. processing.

Thus, high porosity in silica materials leads to a lower dielectricconstant than would otherwise be available from the same materials innonporous form. An additional advantage, is that additional compositionsand processes may be employed to produce nanoporous films while varyingthe relative density of the material. Other materials requirementsinclude the need to have all pores substantially smaller than circuitfeature sizes, the need to manage the strength decrease associated withporosity, and the role of surface chemistry on dielectric constant andenvironmental stability.

Density (or the inverse, porosity) is the key parameter of nanoporousfilms that controls the dielectric constant of the material, and thisproperty is readily varied over a continuous spectrum from the extremesof an air gap at a porosity of 100% to a dense silica with a porosity of0%. As density increases, dielectric constant and mechanical strengthincrease but the degree of porosity decreases, and vice versa. Thissuggests that the density range of nanoporous films must be optimallybalanced between the desired range of low dielectric constant and themechanical properties acceptable for the desired application.

Nanoporous silica films have previously been fabricated by a number ofmethods. For example, nanoporous films have been prepared using amixture of a solvent and a silica precursor, which is deposited on asubstrate suitable for the purpose. Usually, a precursor in the form of,e.g., a spin-on-glass composition is applied to a substrate, and thenpolymerized in such a way as to form a dielectric film comprisingnanometer-scale voids.

When forming such nanoporous films, e.g., by spin-coating, the filmcoating is typically catalyzed with an acid or base catalyst and waterto cause polymerization/gelation (“aging”) during an initial heatingstep. In order to achieve maximum strength through pore size selection,a low molecular weight porogen is used.

U.S. Pat. No. 5,895,263 describes forming a nanoporous silica dielectricfilm on a substrate, e.g., a wafer, by applying a composition comprisingdecomposable polymer and organic polysilica i.e., including condensed orpolymerized silicon polymer, heating the composition to further condensethe polysilica, and decomposing the decomposable polymer to form aporous dielectric layer. This process, like many of the previouslyemployed methods of forming nanoporous films on semiconductors, has thedisadvantage of requiring heating for both the aging or condensingprocess, and for the removal of a polymer to form the nanoporous film.Furthermore, there is a disadvantage that organic polysilica, containedin a precursor solution, tends to increase in molecular weight after thesolution is prepared; consequently, the viscosity of such precursorsolutions increases during storage, and the thickness of films made fromstored solutions will increase as the age of the solution increases. Theinstability of organic polysilica thus requires short shelf life, coldstorage, and fine tuning of the coating parameters to achieve consistentfilm properties in a microelectronics/integrated circuit manufacturingprocess.

Formation of a stable porous structure relies on the condition that theporogen removal temperature is higher than the crosslinking temperature(or the gel temperature) of the matrix material. It was found that astable nanoporous structure of less than 10 nm pore size cannot beproduced when the concentration of the alkali cation such as sodium isbelow 200-300 ppb level in the spin-on solution. However, stringentrequirement for low metal concentration must be met for IC applications.The general practice is to have metal concentration below 50 ppb in thespin-on solution. Therefore, there is a need to develop a low metalnanoporous silica film that can consistently give dielectric constantless than 2.5 and pore size less than about 10 nm in diameter. It hasnow been found that by the use of onium ions or nucleophiles theformation of a porous silica network at lower temperature in a low metalspin-on formulation can be facilitated. The effect of the onium ions ornucleophiles is to lower the gel temperature so that the rigid networkis set in before the removal of the porogen, thus producing a nanoporousfilm without requiring the presence of an undesirable alkali ion.

SUMMARY OF THE INVENTION

The invention provides a method of producing a nanoporous silicadielectric film comprising

-   -   (a) preparing a composition comprising a silicon containing        pre-polymer, a porogen, and a metal-ion-free catalyst selected        from the group consisting of onium compounds and nucleophiles;    -   (b) coating a substrate with the composition to form a film,    -   (c) crosslinking the composition to produce a gelled film, and    -   (d) heating the gelled film at a temperature and for a duration        effective to remove substantially all of said porogen.

The invention also provides a composition comprising silicon containingpre-polymer, a porogen, and a catalyst selected from the groupconsisting of onium compounds and nucleophiles.

The invention further provides a method of lowering the temperature atwhich a porous silica film forms comprising the step of adding oniumions or nucleophiles to a silicon-containing prepolymer and porogen.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows FTIR Spectra for the films of Example 8 wherein the silanolcontent is in the decreasing order: Post Bake Entry 1>>>Post Bake Entry2>Post Cure, Entry 1≠Post Cure Entry 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Accordingly, nanoporous silica dielectric films having a dielectricconstant, or k value, ranging from about 3 or below, can be produced bythe methods of the invention. Typically, silicon-based dielectric films,including nanoporous silica dielectric films, are prepared from acomposition comprising a suitable silicon containing pre-polymer,blended with a porogen and a metal-ion-free catalyst which may be anonium compound or a nucleophile. One or more optional solvents and/orother components may also be included. The dielectric precursorcomposition is applied to a substrate suitable, e.g., for production ofa semiconductor device, such as an integrated circuit (“IC”), by anyart-known method to form a film. The composition is then crosslinked,such as by heating to produce a gelled film. The gelled film is thenheated at a higher temperature to remove substantially all of theporogen.

The films produced by the processes of the invention have a number ofadvantages over those previously known to the art, including improvedmechanical strength, that enables the produced film to withstand thefurther processing steps required to prepare a semiconductor device onthe treated substrate, and a low and stable dielectric constant. Theproperty of a stable dielectric constant is advantageously achievedwithout the need for further surface modification steps to render thefilm surface hydrophobic, as was formerly required by a number ofprocesses for forming nanoporous silica dielectric films. Instead, thesilica dielectric films as produced by the processes of the inventionare sufficiently hydrophobic as initially formed.

Further, the processes of the invention advantageously require arelatively low temperature for the initial polymerization (i.e., gellingor aging) of an applied prepolymer composition. The processes of theinvention provided for a nanometer scale diameter pore size, which isalso uniform in size distribution. The resulting nanoporous silica filmtypically has a dielectric constant of about 3 or below, more typicallyin the range of from about 1.3 to about 3.0, and most typically fromabout 1.7 to about 2.5. The film typically has an average pore diameterranging from about 1 nm to about 30 nm, or more preferably from about 1nm to about 10 nm and typically from about 1 nm to about 5 nm. The filmtypically has a void volume of from about 5% to about 80% based on thetotal volume of the film.

It should be understood that the term nanoporous dielectric films, isintended to refer to dielectric films prepared by the inventive methodsfrom an organic or inorganic glass base material, e.g., any suitablesilicon-based material. Additionally, the term “aging” refers togelling, condensing, or polymerization, of the combined silica-basedprecursor composition on the substrate after deposition. The term“curing” refers to the removal of residual silanol (Si—OH) groups,removal of residual water, and the process of making the film morestable during subsequent processes of the microelectronic manufacturingprocess. The curing process is performed after gelling, typically by theapplication of heat, although any other art-known form of curing may beemployed, e.g., by the application of energy in the form of an electronbeam, ultraviolet radiation, and the like.

Dielectric films, e.g., interlevel dielectric coatings, are preparedfrom suitable compositions applied to a substrate. Art-known methods forapplying the dielectric precursor composition, include, but are notlimited to, spin-coating, dip coating, brushing, rolling, sprayingand/or by chemical vapor deposition. Prior to application of the basematerials to form the dielectric film, the substrate surface isoptionally prepared for coating by standard, art-known cleaning methods.The coating is then processed to achieve the desired type andconsistency of dielectric coating, wherein the processing steps areselected to be appropriate for the selected precursor and the desiredfinal product. Further details of the inventive methods and compositionsare provided below.

A “substrate” as used herein includes any suitable composition formedbefore a nanoporous silica film of the invention is applied to and/orformed on that composition. For example, a substrate is typically asilicon wafer suitable for producing an integrated circuit, and the basematerial from which the nanoporous silica film is formed is applied ontothe substrate by conventional methods, including, but not limited to,the art-known methods of spin-coating, dip coating, brushing, rolling,and/or spraying. Prior to application of the base materials to form thenanoporous silica film, the substrate surface is optionally prepared forcoating by standard, art-known cleaning methods.

Suitable substrates for the present invention non-exclusively includesemiconductor materials such as gallium arsenide (“GaAs”), silicon andcompositions containing silicon such as crystalline silicon,polysilicon, amorphous silicon, epitaxial silicon, and silicon dioxide(“SiO₂”) and mixtures thereof.

On the surface of the substrate is an optional pattern of raised lines,such as metal, oxide, nitride or oxynitride lines which are formed bywell known lithographic techniques. Suitable materials for the linesinclude silica, silicon nitride, titanium nitride, tantalum nitride,aluminum, aluminum alloys, copper, copper alloys, tantalum, tungsten andsilicon oxynitride. Useful metallic targets for making these lines aretaught in commonly assigned U.S. Pat. Nos. 5,780,755; 6,238,494;6,331,233B1; and 6,348,139B1 and are commercially available fromHoneywell International Inc. These lines form the conductors orinsulators of an integrated circuit. Such are typically closelyseparated from one another at distances of about 20 micrometers or less,preferably 1 micrometer or less, and more preferably from about 0.05 toabout 1 micrometer. Other optional features of the surface of a suitablesubstrate include an oxide layer, such as an oxide layer formed byheating a silicon wafer in air, or more preferably, an SiO₂ oxide layerformed by chemical vapor deposition of such art-recognized materials as,e.g., plasma enhanced tetraethoxysilane oxide (“PETEOS”), plasmaenhanced silane oxide (“PE silane”) and combinations thereof, as well asone or more previously formed nanoporous silica dielectric films.

The nanoporous silica film of the invention can be applied so as tocover and/or lie between such optional electronic surface features,e.g., circuit elements and/or conduction pathways that may have beenpreviously formed features of the substrate. Such optional substratefeatures can also be applied above the nanoporous silica film of theinvention in at least one additional layer, so that the low dielectricfilm serves to insulate one or more, or a plurality of electricallyand/or electronically functional layers of the resulting integratedcircuit. Thus, a substrate according to the invention optionallyincludes a silicon material that is formed over or adjacent to ananoporous silica film of the invention, during the manufacture of amultilayer and/or multicomponent integrated circuit. In a furtheroption, a substrate bearing a nanoporous silica film or films accordingto the invention can be further covered with any art known non-porousinsulation layer, e.g., a glass cap layer.

The crosslinkable composition employed for forming nanoporous silicadielectric films according to the invention includes one or more siliconcontaining prepolymers that are readily condensed. It should have atleast 20 two reactive groups that can be hydrolyzed. Such reactivegroups include, alkoxy (RO), acetoxy (AcO), etc. Without being bound byany theory or hypothesis as to how the methods and compositions of theinvention are achieved, it is believed that water hydrolyzes thereactive groups on the silicon monomers to form Si—OH groups (silanols).The latter will undergo condensation reactions with other silanols orwith other reactive groups, as illustrated by the following formulas:Si—OH+HO—Si→Si—O—Si+H₂OSi—OH+RO—Si→Si—O—Si+ROHSi—OH+AcO—Si→Si—O—Si+AcOHSi—OAc+AcO—Si→Si—O—Si+Ac₂O

-   -   R=alkyl or aryl    -   Ac=acyl (CH₃CO)

These condensation reactions lead to formation of silicon containingpolymers. In one embodiment of the invention, the prepolymer includes acompound, or any combination of compounds, denoted by Formula I:Rx-Si-Ly   (Formula I)wherein x is an integer ranging from 0 to about 2 and y is 4−x, aninteger ranging from about 2 to about 4),R is independently alkyl, aryl, hydrogen, alkylene, arylene and/orcombinations of these,L is independently selected and is an electronegative group, e.g.,alkoxy, carboxyl, amino, amido, halide, isocyanato and/or combinationsof these.

Particularly useful prepolymers are those provided by Formula I when xranges from about 0 to about 2, y ranges from about 2 to about 4, R isalkyl or aryl or H, and L is an electronegative group, and wherein therate of hydrolysis of the Si-L bond is greater than the rate ofhydrolysis of the Si—OCH₂CH₃ bond. Thus, for the following reactionsdesignated as (a) and (b):Si-L+H₂O→Si—OH+HL   (a)Si—OCH₂CH₃+H₂O→Si—OH+HOCH₂CH₃   (b)

The rate of (a) is greater than rate of (b).

Examples of suitable compounds according to Formula I include, but arenot limited to:

-   -   Si(OCH₂CF₃)₄ tetrakis(2,2,2-trifluoroethoxy)silane,    -   Si(OCOCF₃)₄ tetrakis(trifluoroacetoxy)silane*,    -   Si(OCN)₄ tetraisocyanatosilane,    -   CH₃Si(OCH₂CF₃)₃ tris(2,2,2-trifluoroethoxy)methylsilane,    -   CH₃Si(OCOCF₃)₃ tris(trifluoroacetoxy)methylsilane*,    -   CH₃Si(OCN)₃ methyltriisocyanatosilane,        [* These generate acid catalyst upon exposure to water]        and or combinations of any of the above.

In another embodiment of the invention, the composition includes apolymer synthesized from compounds denoted by Formula I by way ofhydrolysis and condensation reactions, wherein the number averagemolecular weight ranges from about 150 to about 300,000 amu, or moretypically from about 150 to about 10,000 amu.

In a further embodiment of the invention, silicon-containing prepolymersuseful according to the invention include organosilanes, including, forexample, alkoxysilanes according to Formula II:

Optionally, Formula II is an alkoxysilane wherein at least 2 of the Rgroups are independently C₁ to C₄ alkoxy groups, and the balance, ifany, are independently selected from the group consisting of hydrogen,alkyl, phenyl, halogen, substituted phenyl. For purposes of thisinvention, the term alkoxy includes any other organic groups which canbe readily cleaved from silicon at temperatures near room temperature byhydrolysis. R groups can be ethylene glycoxy or propylene glycoxy or thelike, but preferably all four R groups are methoxy, ethoxy, propoxy orbutoxy. The most preferred alkoxysilanes nonexclusively includetetraethoxysilane (TEOS) and tetramethoxysilane.

In a further option, for instance, the prepolymer can also be analkylalkoxysilane as described by Formula II, but instead, at least 2 ofthe R groups are independently C₁ to C₄ alkylalkoxy groups wherein thealkyl moiety is C₁ to C₄ alkyl and the alkoxy moiety is C₁ to C₆ alkoxy,or ether-alkoxy groups; and the balance, if any, are independentlyselected from the group consisting of hydrogen, alkyl, phenyl, halogen,substituted phenyl. In one preferred embodiment each R is methoxy,ethoxy or propoxy. In another preferred embodiment at least two R groupsare alkylalkoxy groups wherein the alkyl moiety is C₁ to C₄ alkyl andthe alkoxy moiety is C₁ to C₆ alkoxy. In yet another preferredembodiment for a vapor phase precursor, at least two R groups areether-alkoxy groups of the formula (C₁ to C₆ alkoxy)_(n) wherein n is 2to 6.

Preferred silicon containing prepolymers include, for example, any or acombination of alkoxysilanes such as tetraethoxysilane,tetrapropoxysilane, tetraisopropoxysilane, tetra(methoxyethoxy)silane,tetra(methoxyethoxyethoxy)silane which have four groups which may behydrolyzed and than condensed to produce silica, alkylalkoxysilanes suchas methyltriethoxysilane silane, arylalkoxysilanes such asphenyltriethoxysilane and precursors such as triethoxysilane which yieldSiH functionality to the film. Tetrakis(methoxyethoxyethoxy)silane,tetrakis(ethoxyethoxy)silane, tetrakis(butoxyethoxyethoxy)silane,tetrakis(2-ethylthoxy)silane, tetrakis(methoxyethoxy)silane, andtetrakis(methoxypropoxy)silane are particularly useful for theinvention.

In a still further embodiment of the invention, the alkoxysilanecompounds described above may be replaced, in whole or in part, bycompounds with acetoxy and/or halogen-based leaving groups. For example,the prepolymer may be an acetoxy (CH₃—CO—O—) such as an acetoxy-silanecompound and/or a halogenated compound, e.g., a halogenated silanecompound and/or combinations thereof. For the halogenated prepolymersthe halogen is, e.g., Cl, Br, I and in certain aspects, will optionallyinclude F. Preferred acetoxy-derived prepolymers include, e.g.,tetraacetoxysilane, methyltriacetoxysilane and/or combinations thereof.

In one particular embodiment of the invention, the silicon containingprepolymer includes a monomer or polymer precursor, for example,acetoxysilane, an ethoxysilane, methoxysilane and/or combinationsthereof.

In a more particular embodiment of the invention, the silicon containingprepolymer includes a tetraacetoxysilane, a C₁ to about C₆ alkyl oraryl-triacetoxysilane and combinations thereof In particular, asexemplified below, the triacetoxysilane is a methyltriacetoxysilane.

The silicon containing prepolymer is preferably present in the overallcomposition in an amount of from about 10 weight percent to about 80weight percent, preferably present in the overall composition in anamount of from about 20 weight percent to about 60 weight percent.

For non-microelectronic applications, the onium or nucleophile catalystmay contain metal ions. Examples include sodium hydroxide, sodiumsulfate, potassium hydroxide, lithium hydroxide, and zirconiumcontaining catalysts.

For microelectronic applications, preferably, the composition thencontains at least one metal-ion-free catalyst which is an onium compoundor a nucleophile. The catalyst may be, for example an ammonium compound,an amine, a phosphonium compound or a phosphine compound. Non-exclusiveexamples of such include tetraorganoammonium compounds andtetraorganophosphonium compounds including tetramethylammonium acetate,tetramethylammonium hydroxide, tetrabutylammonium acetate,triphenylamine, trioctylamine, tridodecylamine, triethanolamine,tetramethylphosphonium acetate, tetramethylphosphonium hydroxide,triphenylphosphine, trimethylphosphine, trioctylphosphine, andcombinations thereof. The composition may comprise a non-metallic,nucleophilic additive which accelerates the crosslinking of thecomposition.

These include dimethyl sulfone, dimethyl formamide,hexamethylphosphorous triamide (HMPT), amines and combinations thereof.The catalyst is preferably present in the overall composition in anamount of from about 1 ppm by weight to about 1000 ppm, preferablypresent in the overall composition in an amount of from about 6 ppm toabout 200 ppm.

The composition then contains at least one porogen. A porogen may be acompound or oligomer or polymer and is selected so that, when it isremoved, e.g., by the application of heat, a silica dielectric film isproduced that has a nanometer scale porous structure. The scale of thepores produced by porogen removal is proportional to the effectivesteric diameters of the selected porogen component. The need for anyparticular pore size range (i.e., diameter) is defined by the scale ofthe semiconductor device in which the film is employed. Furthermore, theporogen should not be so small as to result in the collapse of theproduced pores, e.g., by capillary action within such a small diameterstructure, resulting in the formation of a non-porous (dense) film.Further still, there should be minimal variation in diameters of allpores in the pore population of a given film. It is preferred thatporogen is a compound that has a substantially homogeneous molecularweight and molecular dimension, and not a statistical distribution orrange of molecular weights,and/or molecular dimensions, in a givensample. The avoidance of any significant variance in the molecularweight distribution allows for a substantially uniform distribution ofpore diameters in the film treated by the inventive processes. If theproduced film has a wide-distribution of pore sizes, the likelihood isincreased of forming one or more large pores, i.e., bubbles, that couldinterfere with the production of reliable semiconductor devices.

Furthermore, the porogen should have a molecular weight and structuresuch that it is readily and selectively removed from the film withoutinterfering with film formation. This is based on the nature ofsemiconductor devices, which typically have an upper limit to processingtemperatures. Broadly, a porogen should be removable from the newlyformed film at temperatures below, e.g., about 450° C. In particularembodiments, depending on the desired post film formation fabricationprocess and materials, the porogen is selected to be readily removed attemperatures ranging from about 150° C. to about 450° C. during a timeperiod ranging, e.g., from about 30 seconds to about 60 minutes. Theremoval of the porogen may be induced by heating the film at or aboveatmospheric pressure or under a vacuum, or by exposing the film toradiation, or both.

Porogens which meet the above characteristics include those compoundsand polymers which have a boiling point, sublimation temperature, and/ordecomposition temperature (at atmospheric pressure) range, for example,from about 150° C. to about 450° C. In addition, porogens suitable foruse according to the invention include those having a molecular weightranging, for example, from about 100 to about 50,000 amu, and morepreferably in the range of from about 100 to about 3,000 amu.

Porogens suitable for use in the processes and compositions of theinvention include polymers, preferably those which contain one or morereactive groups, such as hydroxyl or amino. Within these generalparameters, a suitable polymer porogen for use in the compositions andmethods of the invention is, e.g., a polyalkylene oxide, a monoether ofa polyalkylene oxide, a diether of a polyalkylene oxide, bisether of apolyalkylene oxide, an aliphatic polyester, an acrylic polymer, anacetal polymer, a poly(caprolactone), a poly(valeractone), a poly(methylmethacrylate), a poly (vinylbutyral) and/or combinations thereof. Whenthe porogen is a polyalkylene oxide monoether, one particular embodimentis a C₁ to about C₆ alkyl chain between oxygen atoms and a C₁ to aboutC₆ alkyl ether moiety, and wherein the alkyl chain is substituted orunsubstituted, e.g., polyethylene glycol monomethyl ether, polyethyleneglycol dimethyl ether, or polypropylene glycol monomethyl ether.

Other useful porogens are disclosed in commonly assigned patentapplication Ser. No. ______ filed on the same day as this application,are porogens that do not bond to the silicon containing pre-polymer, andinclude a poly(alkylene) diether, a poly(arylene) diether, poly(cyclicglycol) diether, Crown ethers, polycaprolactone, fully end-cappedpolyalkylene oxides, fully end-capped polyarylene oxides, polynorbene,and combinations thereof. Preferred porogens which do not bond to thesilicon containing pre-polymer include poly(ethylene glycol) dimethylethers, poly(ethylene glycol) bis(carboxymethyl) ethers, poly(ethyleneglycol) dibenzoates, poly(ethylene glycol) diglycidyl ethers, apoly(propylene glycol) dibenzoates, poly(propylene glycol) diglycidylethers, poly(propylene glycol) dimethyl ether, 15-Crown 5, 18-Crown-6,dibenzo-18-Crown-6, dicyclohexyl-18-Crown-6, dibenzo-1 5-Crown-5 andcombinations thereof.

Without meaning to be bound by any theory or hypothesis as to how theinvention might operate, it is believed that porogens that are, “readilyremoved from the film” undergo one or a combination of the followingevents: (1) physical evaporation of the porogen during the heating step,(2) degradation of the porogen into more volatile molecular fragments,(3) breaking of the bond(s) between the porogen and the Si containingcomponent, and subsequent evaporation of the porogen from the film, orany combination of modes 1-3. The porogen is heated until a substantialproportion of the porogen is removed, e.g., at least about 50% byweight, or more, of the porogen is removed. More particularly, incertain embodiments, depending upon the selected porogen and filmmaterials, at least about 75% by weight, or more, of the porogen isremoved.

Thus, by “substantially” is meant, simply by way of example, removingfrom about 50% to about 75%, or more, of the original porogen from theapplied film.

A porogen is preferably present in the overall composition, in an amountranging from about 1 to about 50 weight percent, or more. Morepreferably the porogen is present in the composition, in an amountranging from about 2 to about 20 weight percent.

The overall composition then optionally includes a solvent composition.Reference herein to a “solvent” should be understood to encompass asingle solvent, polar or nonpolar and/or a combination of compatiblesolvents forming a solvent system selected to solubilize the overallcomposition components. A solvent is optionally included in thecomposition to lower its viscosity and promote uniform coating onto asubstrate by art-standard methods (e.g., spin coating, spray coating,dip coating, roller coating, and the like).

In order to facilitate solvent removal, the solvent is one which has arelatively low boiling point relative to the boiling point of anyselected porogen and the other precursor components. For example,solvents that are useful for the processes of the invention have aboiling point ranging from about 50 to about 250° C. to allow thesolvent to evaporate from the applied film and leave the active portionof the precursor composition in place. In order to meet various safetyand environmental requirements, the solvent preferably has a high flashpoint (generally greater than 40° C.) and relatively low levels oftoxicity. A suitable solvent includes, for example, hydrocarbons, aswell as solvents having the functional groups C—O—C (ethers), —CO—O(esters), —CO— (ketones), —OH (alcohols), and CO—N-(amides), andsolvents which contain a plurality of these functional groups, andcombinations thereof.

Without limitation, solvents for the composition include di-n-butylether, anisole, acetone, 3-pentanone, 2-heptanone, ethyl acetate,n-propyl acetate, n-butyl acetate, ethyl lactate, ethanol, 2-propanol,dimethyl acetamide, propylene glycol methyl ether acetate, and/orcombinations thereof. It is preferred that the solvent does not reactwith the silicon containing prepolymer component.

The solvent component is preferably present in an amount of from about10% to about 95% by weight of the overall composition. A more preferredrange is from about 20% to about 75% and most preferably from about 20%to about 60%. The greater the percentage of solvent employed, thethinner is the resulting film. The greater the percentage of porogenemployed, the greater is the resulting porosity.

In another embodiment of the invention the composition may compriseswater, either liquid or water vapor. For example, the overallcomposition may be applied to a substrate and then exposed to an ambientatmosphere that includes water vapor at standard temperatures andstandard atmospheric pressure. Optionally, the composition is preparedprior to application to a substrate to include water in a proportionsuitable for initiating aging of the precursor composition, withoutbeing present in a proportion that results in the precursor compositionaging or gelling before it can be applied to a desired substrate. By wayof example, when water is mixed into the precursor composition it ispresent in a proportion wherein the composition comprises water in amolar ratio of water to Si atoms in the silicon containing prepolymerranging from about 0.1:1 to about 50:1. A more preferred range is fromabout 0.1:1 to about 10:1 and most preferably from about 0.5:1 to about1.5:1.

Those skilled in the art will appreciate that specific temperatureranges for crosslinking and porogen removal from the nanoporousdielectric films will depend on the selected materials, substrate anddesired nanoscale pore structure, as is readily determined by routinemanipulation of these parameters. Generally, the coated substrate issubjected to a treatment such as heating to effect crosslinking of thecomposition on the substrate to produce a gelled film.

Crosslinking may be done in step (c) by heating the film at atemperature ranging from about 100° C. to about 250° C., for a timeperiod ranging from about 30 seconds to about 10 minutes to gel thefilm. The artisan will also appreciate that any number of additionalart-known curing methods are optionally employed. including theapplication of sufficient energy to cure the film by exposure of thefilm to electron beam energy, ultraviolet energy, microwave energy, andthe like, according to art-known methods.

Once the film has aged, i.e., once it is is sufficiently condensed to besolid or substantially solid, the porogen can be removed. The lattershould be sufficiently non-volatile so that it does not evaporate fromthe film before the film solidifies. The porogen is removed in a step(d) by heating the gelled film at a temperature ranging from about 150°C. to about 450° C., preferably from about 150° C. to about 350° C. fora time period ranging from about 30 seconds 20 to about 1 hour. Animportant feature of the invention is that preferably the step (c)crosslinking is conducted at a temperature which is less than theheating temperature of step (d).

Utility:

The present composition may also comprise additional components such asadhesion promoters, antifoam agents, detergents, flame retardants,pigments, plasticizers, stabilizers, and surfactants. The presentcomposition has utility in non-microelectronic applications such asthermal insulation, encapsulant, matrix materials for polymer andceramic composites, light weight composites, acoustic insulation,anti-corrosive coatings, binders for ceramic powders, and fire retardantcoatings.

The present composition is particularly useful in microelectronicapplications as a dielectric substrate material in microchips, multichipmodules, laminated circuit boards, or printed wiring boards. The presentcomposition may also be used as an etch stop or hardmask.

The present films may be formed by solution techniques such as spraying,10 rolling, dipping, spin coating, flow coating, or casting, andchemical vapor deposition, with spin coating being preferred formicroelectronics. For chemical vapor deposition (CVD), the compositionis placed into an CVD apparatus, vaporized, and introduced into adeposition chamber containing the substrate to be coated. Vaporizationmay be accomplished by heating the composition above its vaporizationpoint, by the use of vacuum, or by a combination of the above.Generally, vaporization is accomplished at temperatures in the range of50° C.-300° C. under atmospheric pressure or at lower temperature (nearroom temperature) under vacuum.

Three types of CVD processes exist: atmospheric pressure CVD (APCVD),low pressure CVD (LPCVD), and plasma enhanced CVD (PECVD). Each of theseapproaches had advantages and disadvantages. APCVD devices operate in amass transport limited reaction mode at temperatures of approximately400° C. In mass-transport limited deposition, temperature control of thedeposition chamber is less critical than in other methods because masstransport processes are only weakly dependent on temperature. As thearrival rate of the reactants is directly proportional to theirconcentration in the bulk gas, maintaining a homogeneous concentrationof reactants in the bulk gas adjacent to the wafers is critical. Thus,to insure films of uniform thickness across a wafer, reactors that areoperated in the mass transport limited regime must be designed so thatall wafer surfaces are supplied with an equal flux of reactant. The mostwidely used APCVD reactor designs provide a uniform supply of reactantsby horizontally positioning the wafers and moving them under a gasstream.

In contrast to APCVD reactors, LPCVD reactors operate in a reactionrate-limited mode. In processes that are run under reaction rate-limitedconditions, the temperature of the process is an important parameter. Tomaintain a uniform deposition rate throughout a reactor, the reactortemperature must be homogeneous throughout the reactor and at all wafersurfaces. Under reaction rate-limited conditions, the rate at which thedeposited species arrive at the surface is not as critical as constanttemperature. Thus, LPCVD reactors do not have to be designed to supplyan invariant flux of reactants to all locations of a wafer surface.

Under the low pressure of an LPCVD reactor, for example, operating atmedium vacuum (30-250 Pa or 0.25-2.0 torr) and higher temperature(550-600° C.), the diffusivity of the deposited species is increased bya factor of approximately 1000 over the diffusivity at atmosphericpressure. The increased diffusivity is partially offset by the fact thatthe distance across which the reactants must diffusive increases by lessthan the square root of the pressure. The net effect is that there ismore than an order of magnitude increase in the transport of reactantsto the substrate surface and by-products away from the substratesurface.

LPCVD reactors are designed in two primary configurations: (a)horizontal tube reactors; and (b) vertical flow isothermal reactors.Horizontal tube, hot wall reactors are the most widely used LPCVDreactors in VLSI processing. They are employed for depositing poly-Si,silicon nitride, and undoped and doped SiO₂ films. They find such broadapplicability primarily because of their superior economy, throughput,uniformity, and ability to accommodate large diameter, e.g., 150 mm,wafers.

The vertical flow isothermal LPCVD reactor further extends thedistributed gas feed technique so that each wafer receives an identicalsupply of fresh reactants. Wafers are again stacked side by side, butare placed in perforated-quartz cages. The cages are positioned beneathlong, perforated, quartz reaction-gas injector tubes, one tube for eachreactant gas. Gas flows vertically from the injector tubes, through thecage perforations, past the wafers, parallel to the wafer surface andinto exhaust slots below the cage. The size, number, and location ofcage perforations are used to control the flow of reactant gases to thewafer surfaces. By properly optimizing cage perforation design, eachwafer may be supplied with identical quantities of fresh reactants fromthe vertically adjacent injector tubes. Thus, this design may avoid thewafer-to-wafer reactant depletion effects of the end-feed tube reactors,requires no temperature ramping, produces highly uniform depositions,and reportedly achieves low particulate contamination.

The third major CVD deposition method is PECVD. This method iscategorized not only by pressure regime, but also by its method ofenergy input. Rather than relying solely on thermal energy to initiateand sustain chemical reactions, PECVD uses an rf-induced glow dischargeto transfer energy into the reactant gases, allowing the substrate toremain at a lower temperature than in APCVD or LPCVD processes. Lowersubstrate temperature is the major advantages of PECVD, providing filmdeposition on substrates not having sufficient thermal stability toaccept coating by other methods. PECVD may also enhance deposition ratesover those achieved using thermal reactions. Moreover, PECVD may producefilms having unique compositions and properties. Desirable propertiessuch as good adhesion, low pinpole density, good step coverage, adequateelectrical properties, and compatibility with fine-line pattern transferprocesses, have led to application of these films in VLSI.

PECVD requires control and optimization of several depositionparameters, including rf power density, frequency, and duty cycle. Thedeposition process is dependent in a complex and interdependent way onthese parameters, as well as on the usual parameters of gas composition,flow rates, temperature, and pressure. Furthermore, as with LPCVD, thePECVD method is surface reaction limited, and adequate substratetemperature control is thus necessary to ensure uniform film thickness.

CVD systems usually contain the following components: gas sources, gasfeed lines, mass-flow controllers for metering the gases into thesystem, a reaction chamber or reactor, a method for heating the wafersonto which the film is to be deposited, and in some types of systems,for adding additional energy by other means, and temperature sensors.LPCVD and PECVD systems also contain pumps for establishing the reducedpressure and exhausting the gases from the chamber.

Preferably, the present composition is dissolved in a solvent. Suitablesolvents for use in such solutions of the present compositions includeany suitable pure or mixture of organic, organometallic, or inorganicmolecules that are volatized at a desired temperature. Suitable solventsinclude aprotic solvents, for example, cyclic ketones such ascyclopentanone, cyclohexanone, cycloheptanone, and cyclooctanone; cyclicamides such as N-alkylpyrrolidinone wherein the alkyl has from about 1to 4 carbon atoms; and N-cyclohexylpyrrolidinone and mixtures thereof. Awide variety of other organic solvents may be used herein insofar asthey are able to aid dissolution of the adhesion promoter and at thesame time effectively control the viscosity of the resulting solution asa coating solution. Various facilitating measures such as stirringand/or heating may be used to aid in the dissolution. Other suitablesolvents include methyethylketone, methylisobutylketone, dibutyl ether,cyclic dimethylpolysiloxanes, butyrolactone, γ-butyrolactone,2-heptanone, ethyl 3-ethoxypropionate, 1-methyl-2-pyrrolidinone, andpropylene glycol methyl ether acetate (PGMEA), and hydrocarbon solventssuch as mesitylene, xylenes, benzene, and toluene.

The present composition may be used in electrical devices and morespecifically, as an interlayer dielectric in an interconnect associatedwith a single integrated circuit (“IC”) chip. An integrated circuit chiptypically has on its surface a plurality of layers of the presentcomposition and multiple layers of metal conductors. It may also includeregions of the present composition between discrete metal conductors orregions of conductor in the same layer or level of an integratedcircuit.

The present films may be formed on substrates. Substrates contemplatedherein may comprise any desirable substantially solid material.Particularly desirable substrate layers comprise films, glass, ceramic,plastic, metal or coated metal, or composite material. In preferredembodiments, the substrate comprises a silicon or gallium arsenide dieor wafer surface, a packaging surface such as found in a copper, silver,nickel or gold plated leadframe, a copper surface such as found in acircuit board or package interconnect trace, a via-wall or stiffenerinterface (“copper” includes considerations of bare copper and itsoxides), a polymer-based packaging or board interface such as found in apolyimide-based flex package, lead or other metal alloy solder ballsurface, glass and polymers. Useful substrates include silicon, siliconnitride, silicon oxide, silicon oxycarbide, silicon dioxide, siliconcarbide, silicon oxynitride, titanium nitride, tantalum nitride,tungsten nitride, aluminum, copper, tantalum, organosiloxanes, organosilicon glass, and fluorinated silicon glass. In other embodiments, thesubstrate comprises a material common in the packaging and circuit boardindustries such as silicon, copper, glass, and polymers. The circuitboard made up of the present composition will have mounted on itssurface patterns for various electrical conductor circuits. The circuitboard may include various reinforcements, such as woven non-conductingfibers or glass cloth. Such circuit boards may be single sided, as wellas double sided.

The present films may be used in dual damascene (such as copper)processing and substractive metal (such as aluminum oraluminum/tungsten) processing for integrated circuit manufacturing. Thepresent composition may be used in a desirable all spin-on stacked filmas taught by Michael E. Thomas, “Spin-On Stacked Films for Low k_(eff)Dielectrics”, Solid State Technology (July 2001), incorporated herein inits entirety by reference. The present composition may be used in an allspin-on stacked film having additional dielectrics such as taught bycommonly assigned U.S. Pat. Nos. 6,248,457B1; 5,986,045; 6,124,411; and6,303,733.

Analytical Test Methods:

Dielectric Constant: The dielectric constant was determined by coating athin film of aluminum on the cured layer and then doing acapacitance-voltage measurement at 1 MHz and calculating the k valuebased on the layer thickness.

Refractive Index: The refractive index measurements were performedtogether with the thickness measurements using a J. A. Woollam M-88spectroscopic ellipsometer. A Cauchy model was used to calculate thebest fit for Psi and Delta. Unless noted otherwise, the refractive indexwas reported at a wavelength of 633 nm (details on Ellipsometry can befound in e.g. “Spectroscopic Ellipsometry and Reflectometry” by H. G.Thompkins and William A. McGahan, John Wiley and Sons, Inc., 1999).

Average Pore Size Diameter: The N₂ isotherms of porous samples wasmeasured on a Micromeretics ASAP 2000 automatic isothermal N₂ sorptioninstrument using UHP (ultra high purity industrial gas) N₂, with thesample immersed in a sample tube in a liquid N₂ bath at 77° K.

For sample preparation, the material was first deposited on siliconwafers using standard processing conditions. For each sample, threewafers were prepared with a film thickness of approximately 6000Angstroms. The films were then removed from the wafers by scraping witha razor blade. These powder samples were pre-dried at 180° C. in an ovenbefore weighing them, carefully pouring the powder into a 10 mm innerdiameter sample tube, then degassing at 180° C. at 0.01 Torr for >3hours.

The adsorption and desorption N₂ sorption was then measuredautomatically using a 5 second equilibration interval, unless analysisshowed that a longer time was required. The time required to measure theisotherm was proportional to the mass of the sample, the pore volume ofthe sample, the number of data points measured, the equilibrationinterval, and the P/Po tolerance. (P is the actual pressure of thesample in the sample tube. Po is the ambient pressure outside theinstrument.) The instrument measures the N₂ isotherm and plots N₂ versusP/Po.

The apparent BET (Brunauer, Emmett, Teller method for multi-layer gasabsorption on a solid surface disclosed in S. Brunauer, P. H. Emmett, E.Teller; J. Am. Chem. Soc. 60, 309-319 (1938)) surface area wascalculated from the lower P/Po region of the N2 adsorption isothermusing the BET theory, using the linear section of the BET equation thatgives an R² fit >0.9999.

The pore volume was calculated from the volume of N₂ adsorbed at therelative pressure P/Po value, usually P/Po˜0.95, which is in the flatregion of the isotherm where condensation is complete, assuming that thedensity of the adsorbed N₂ is the same as liquid N₂ and that all thepores are filled with condensed N₂ at this P/Po.

The pore size distribution was calculated from the adsorption arm of theN₂ isotherm using the BJH (E. P. Barret, L. G. Joyner, P. P. Halenda; J.Am. Chem. Soc., 73, 373-380 (1951)) pore size distribution from the N2isotherm using the Kelvin equation theory. This uses the Kelvinequation, which relates curvature to suppression of vapor pressure, andthe Halsey equation, which describes the thickness of the adsorbed N₂monolayer versus P/Po, to convert the volume of condensed N₂ versus P/Poto the pore volume in a particular range of pore sizes.

The average cylindrical pore diameter D was the diameter of a cylinderthat has the same apparent BET surface area Sa (m²/g) and pore volume Vp(cc/g) as the sample, so D (nm)=4000 Vp/Sa.

The following non-limiting examples serve to illustrate the invention.

EXAMPLE 1

This example shows the production of a nanoporous silica with a porogenhaving a high concentration of sodium. A precursor was prepared bycombining, in a 100 ml round bottom flask (containing a magneticstirring bar), 10 g tetraacetoxysilane, 10 g methyltriacetoxysilane, and17 g propylene glycol methyl ethyl acetate (PGMEA). These ingredientswere combined within an N₂-environment (N₂ glove bag). The flask wasalso connected to an N₂ environment to prevent environmental moisturefrom entering the solution (standard temperature and pressure).

The reaction mixture was heated to 80° C. before 1.5 g of water wasadded to the flask. After the water addition is complete, the reactionmixture was allowed to cool to ambient before 4.26 g of polyethyleneglycol monomethylether (“PEO”; MW550 amu) (with >300 ppb Na) was addedas a porogen, and stirring continued for another 2 hrs. Thereafter, theresulting solution was filtered through a 0.2 micron filter to providethe precursor solution masterbatch for the next step. The solution isthen deposited onto a series of 8-inch silicon wafers, each on a spinchuck and spun at 2500 rpm for 30 seconds. The presence of water in theprecursor resulted in the film coating being substantially condensed bythe time that the wafer was inserted into the first oven. Insertion intothe first oven, as discussed below, takes place within the 10 seconds ofthe completion of spinning. Each coated wafer was then transferred intoa sequential series of ovens preset at specific temperatures, for oneminute each. In this example, there are three ovens, and the preset oventemperatures were 80° C., 175° C., and 300° C., respectively. The PEO isdriven off by these sequential heating steps as each wafer was movedthrough each of the three respective ovens. Each wafer is cooled afterreceiving the three-oven stepped heat treatment, and the produceddielectric film was measured using ellipsometry to determine itsthickness and refractive index. Each film-coated wafer is then furthercured at 425° C. for one hour under flowing nitrogen. A non-porous filmmade from the liquid precursor of this invention will have a refractiveindex of 1.41 and a k de-gas of 3.2. In comparison, air has a refractiveindex of 1.0. The porosity of a nanoporous film of the invention, istherefore proportional to the percentage of its volume that is air. Thefilm has a bake thickness of 5920 Å, a bake refractive index of 1.234, acure thickness of 5619 Å and a cure refractive index of 1.231. The curedfilm produced has a porosity of about 43% (see entry 1 of the followingtable). In the table, capacitance of the film was measured under ambientconditions (room temperature and humidity). Dielectric constant based onambient capacitance value is called k ambient. The capacitance of thefilm was measured again after heating the wafer in a hot plate at 200Cfor 2 minutes in order to drive off adsorbed moisture. Dielectricconstant based on the de-moisture capacitance is called k de-gas.

EXAMPLE 2 (COMPARATIVE)

This example shows the production of a nanoporous silica with a porogenhaving a low concentration of sodium.

Crude PEO (polyethylene glycol monomethyl ether MW=550) with highconcentration of sodium was purified by mixing the crude PEO with waterin a 50:50 weight ratio. This mixture was passed through an ion exchangeresin to remove metals. The filtrate was collected and subjected tovacuum distillation to remove water to produce neat, low metal PEO(with<100 ppb Na). The procedure of Example 1 was then followed with the lowmetal PEO substituted for the high metal PEO. It is estimated that froma k de-gas value of 3.03, the film basically collapses and has aporosity of only about 7%, a drop from 43% as compared to Example 1(Comparative). The film has a bake thickness of 4179 Å, a bakerefractive index of 1.353, a cure thickness of 3875 Å and a curerefractive index of 1.331 (see entry 2 of the following table).

EXAMPLE 3

Example 2 is repeated except this example adds sodium cation (sodiumhydroxide (see entry 3), or sodium sulfate (see entry 4) to restore thelow k. Sodium hydroxide (NaOH, 23 ppm) or sodium sulfate (Na₂SO₄, 40ppm) was were added to the ion-exchanged PEO and the precursormasterbatch. Films are deposited onto a wafer by spin coating at 2400rpm or 3500 rpm. After spin coating, the film is heated in three hotplates at temperatures of 80° C., 175° C. and 300° C., one minute each.After bake, the film is cured under flowing nitrogen at 425° C. for onehour. The results of k and R.I. of the post-cure films are listed in thefollowing table.

EXAMPLE 4

Example 2 is repeated except this example adds tetraorganoammonium(TMAA) (entries 5, 6, and 7), TMAH (entry 8), or TBAA (entry 9)) ion torestore the low k. Various amounts of TMAA were added to theion-exchanged PEO and the precursor masterbatch. In some cases, smallamount of methyltriacetoxysilane (MTAS, 1%, see entry 6) were added tothe solution to serve as an in-situ surface modifier to make the surfaceless hydrophilic. Films were deposited onto a wafer by spin coating at2400 rpm or 3500 rpm. After spin coating, the film was heated in threehot plates at temperatures of 80° C., 175° C. and 300° C., one minuteeach. After bake, the film was cured under flowing nitrogen at 425° C.for one hour. For entries 5 and 6, the average pore size diameter was2.5 nm. The results of k and R.I. of the post-cure films are listed inthe following table. It is shown that the k value is below 2.5 when theconcentration of the ammonium ion is greater than about 65×10⁻⁹ mole/gmof solution, which corresponds to approximately 3 ppm of TMAA by weight.

EXAMPLE 5

This example shows the production of a nanoporous silica prepared from aporogen having a low concentration of sodium, and a commerciallyavailable methylsiloxane polymer (Honeywell ACCUGLASS® SPIN-ON GLASS512B).

Crude PEO (polyethylene glycol monomethyl ether MW=550) with highconcentration of sodium is purified by mixing the crude PEO with waterin a 50:50 weight ratio. This mixture is passed through an ion exchangeresin to remove metals. The filtrate is collected and subjected tovacuum distillation to remove water to produce neat, low metal PEO(with<100 ppb Na). The resulting PEO (4.88 g) and butanol (48 g) are mixed inwith ACCUGLASS® SPIN-ON GLASS 512B (43 g). Thereafter, the resultingsolution is filtered through a 0.2 micron filter to provide theprecursor solution masterbatch for the next step. The solution is thendeposited onto a series of 8-inch silicon wafers, each on a spin chuckand spun at 3000 rpm for 30 seconds. The presence of water in theprecursor resulted in the film coating being substantially condensed bythe time that the wafer was inserted into the first oven. Insertion intothe first oven, as discussed below, takes place within the 10 seconds ofthe completion of spinning. Each coated wafer is then transferred into asequential series of ovens preset at specific temperatures, for oneminute each. In this example, there are three ovens, and the preset oventemperatures were 80° C., 175° C., and 300° C., respectively. The PEO isdriven off by these sequential heating steps as each wafer was movedthrough each of the three respective ovens. Each wafer is cooled afterreceiving the three-oven stepped heat treatment, and the produceddielectric film was measured using ellipsometry to determine itsthickness and refractive index. Each film-coated wafer is then furthercured at 425° C. for one hour under flowing Nitrogen. The film collapsesand cannot form a porous structure. The film has a bake thickness of1690 Å, a bake refractive index of 1.395, a cure thickness of 1615 Å anda cure refractive index of 1.367. The cured film produced has a porosityof about 5% (see entry 10 of the following table).

EXAMPLE 6

Example 5 was repeated except this example adds tetraorganoammonium(TMAA (entry 11) ion to restore the low k. TMAA (10 ppm) is added to theion-exchanged PEO (3.64 g), butanol (13 g) and ACCUGLASS® SPIN-ON GLASS512B (25 g). Films were deposited onto a wafer by spin coating at 2000rpm. After spin coating, the film was heated in three hot plates attemperatures of 125° C., 200° C. and 350° C., one minute each. Afterbake, the film was cured under flowing nitrogen at 425° C. for one hour.The results of k and R.I. of the post-cure films are listed in thefollowing table. The cured film produced has a porosity of about 40%.The average pore size diameter was 2.5 nm. TABLE I Additive Final Conc.Na Cured Cured Entry Ppm PEO k_(ambient) K_(de-gas) Δk Ppb R.I.Thickness Å 1  0 Crude 2.60 2.36 0.24 285 1.231 5619 2  0 Low 3.62 3.030.59 <25 1.331 3875 metal 3  40 (Na₂SO₄) Low 2.21 2.10 0.11 1.232 7244metal 4  23 (NaOH) Low 2.24 2.21 0.03 1.246 5479 Metal 5  6 (TMAA) Low2.34 2.23 0.11 43 1.209 6127 metal 6  6 (TMAA) Low 2.24 2.13 0.11 431.218 6319 metal 7  3 (TMAA) Low 2.65 2.38 0.27 95 1.248 6495 metal 8 22 (TMAH) Low 2.27 2.10 0.17 <25 1.215 7871 metal 9 100 (TBAA) Low 2.552.27 0.28 65 1.213 7716 metal 10  0 Low n/a N/a n/a <25 1.367 1615 metal11  10 (TMAA) Low 2.41 2.24 0.17 <25 1.215 6467 Metal

EXAMPLE 7

The following example (entry 1 of table II) shows the condensation (alsoknown as cross-linking of silanol groups) reaction is impaired in theabsence of TMAA at 300° C. For illustrative purpose, porogen is notadded. A precursor was prepared by combining, in a 100 ml round bottomflask (containing a magnetic stirring bar), 10 g tetraacetoxysilane, 10g methyltriacetoxysilane, and 17 g propylene glycol methyl ethyl acetate(PGMEA). These ingredients were combined within an N₂-environment (N₂glove bag). The flask was also connected to an N₂ environment to preventenvironmental moisture from entering the solution (standard temperatureand pressure). The reaction mixture was heated to 80° C. before 1.5 g ofwater was added to the flask. After the water addition is complete, thereaction mixture was allowed to cool to ambient before the resultingsolution was filtered through a 0.2 micron filter to provide theprecursor solution masterbatch for the next step. The solution is thendeposited onto a series of 8-inch silicon wafers, each on a spin chuckand spun at 2500 rpm for 30 seconds. The presence of water in theprecursor resulted in the film coating being substantially condensed bythe time that the wafer was inserted into the first hot-plate. Insertioninto the first hot-plate, as discussed below, takes place within the 10seconds of the completion of spinning. Each coated wafer was thentransferred into a sequential series of hot-plates preset at specifictemperatures, for one minute each. In this example, there are threehot-plates, and the preset hot-plate temperatures were 80° C., 175° C.,and 300° C., respectively. Each wafer is cooled after receiving thethree-hot-plate stepped heat treatment, and the produced dielectric filmwas measured using ellipsometry to determine its thickness andrefractive index, and FTIR to measure the silanol(SiOH, ν: 3100-3800cm⁻¹)-to-methyl(CH₃, ν: 2978 cm⁻¹) area ratio. The observed (Si)OH toCH₃ is greater than 20 for the 3 kÅ film with RI of 1.41±0.01. The FTIRspectrum of the baked film illustrates the large amount of silanol. (SeeFIG. 1) Each film-coated wafer is then further cured at 425° C. for onehour under flowing nitrogen. The resulting film has a (Si)OH to CH₃ratio of 2.

EXAMPLE 8

Example 7 was repeated except this example adds tetraorganoammonium(TMAA) (entry 2). This example illustrates the condensation reaction at300° C. is catalyzed by the presence of TMAA. Films were deposited ontoa wafer by spin coating at 2400 rpm. After spin coating, the film washeated in three hot plates at temperatures of 80° C., 175° C. and 300°C., one minute each. After bake, the film was analyzed by FTIR todetermine the silanol(SiOH)-to-methyl(CH₃) area ratio. The observed(Si)OH to CH₃ ratio was ca. 4 for the 3 kÅ film with RI of 1.41. Thedecrease in silanol content is better illustrated by the FTIR spectrumof the film. (See FIG. 1). FIG. 1 shows FTIR spectra where the silanolcontent is in the decreasing order: Post Bake Entry 1>>>Post Bake Entry2>Post Cure, Entry 1≈Post Cure Entry 2.

Each film-coated wafer was then further cured at 425° C. for one hourunder flowing nitrogen. The resulting film has a (Si)OH to CH₃ ratio of2. TABLE II Baked Cured TMAA Conc. SiO—H/ SiO—H/ Entry Ppm RI SiC—H₃ RISiC—H₃ 1 0 1.41 22 1.41 2 2 5 1.41  4 1.41 2

While the present invention has been particularly shown and describedwith reference to preferred embodiments, it will be readily appreciatedby those of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. It is intended that the claims be interpreted to coverthe disclosed embodiment, those alternatives which have been discussedabove and all equivalents thereto.

1. A method of producing a nanoporous silica dielectric film comprising(a) preparing a composition comprising a silicon containing pre-polymer,a porogen, and a metal-ion-free catalyst selected from the groupconsisting of onium compounds and nucleophiles; (b) coating a substratewith the composition to form a film, (c) crosslinking the composition toproduce a gelled film, and (d) heating the gelled film at a temperatureand for a duration effective to remove substantially all of saidporogen.
 2. The method of claim 1 wherein the nanoporous silicadielectric film has a pore void volume of from about 5% to about 80%based on the volume of the film.
 3. The method of claim 1 wherein theresulting nanoporous silica dielectric film has a dielectric constant ofabout 3 or below.
 4. The method of claim 1 wherein the nanoporous silicadielectric film has an average pore diameter in the range of from about1 nm to about 30 nm.
 5. The method of claim 1 wherein the catalyst isselected from the group consisting of ammonium compounds, amines,phosphonium compounds and phosphine compounds.
 6. The method of claim 1wherein the catalyst is selected from the group consisting oftetraorganoammonium compounds and tetraorganophosphonium compounds. 7.The method of claim 1 wherein the catalyst is selected from the groupconsisting of tetramethylammonium acetate, tetramethylammoniumhydroxide, tetrabutylammonium acetate, triphenylamine, trioctylamine,tridodecylamine, triethanolamine, tetramethylphosphonium acetate,tetramethylphosphonium hydroxide, triphenylphosphine,trimethylphosphine, trioctylphosphine, and combinations thereof.
 8. Themethod of claim 1 wherein the composition further comprises anon-metallic, nucleophilic additive which accelerates the crosslinkingof the composition.
 9. The method of claim 1 wherein the compositionfurther comprises a nucleophilic additive which accelerates thecrosslinking of the composition, which is selected from the groupconsisting of dimethyl sulfone, dimethyl formamide,hexamethylphosphorous triamide, amines and combinations thereof.
 10. Themethod of claim 1wherein the composition further comprises water in amolar ratio of water to Si ranging from about 0.1:1 to about 50:1. 11.The method of claim 1 wherein the composition comprises a siliconcontaining prepolymer of Formula I:Rx-Si-Ly   (Formula I) wherein x is an integer ranging from 0 to about2, and y is x−4, an integer ranging from about 2 to about 4; R isindependently selected from the group consisting of alkyl, aryl,hydrogen, alkylene, arylene, and combinations thereof; L is anelectronegative moiety, independently selected from the group consistingof alkoxy, carboxyl, acetoxy, amino, amido, halide, isocyanato andcombinations thereof.
 12. The method of claim 11 wherein the compositioncomprises a polymer formed by condensing a prepolymer according toFormula I, wherein the number average molecular weight of said polymerranges from about 150 to about 300,000 amu.
 13. The method of claim 1wherein the composition comprises a silicon containing pre-polymerselected from the group consisting of an acetoxysilane, an ethoxysilane,a methoxysilane, and combinations thereof.
 14. The method of claim 1wherein the composition comprises a silicon containing pre-polymerselected from the group consisting of tetraacetoxysilane, a C₁ to aboutC₆ alkyl or aryl-triacetoxysilane, and combinations thereof.
 15. Themethod of claim 14 wherein said triacetoxysilane ismethyltriacetoxysilane.
 16. The method of claim 1 wherein thecomposition comprises a silicon containing pre-polymer selected from thegroup consisting of tetrakis(2,2,2-trifluoroethoxy)silane,tetrakis(trifluoroacetoxy)silane, tetraisocyanatosilane,tris(2,2,2-trifluoroethoxy)methylsilane,tris(trifluoroacetoxy)methylsilane, methyltriisocyanatosilane andcombinations thereof.
 17. The method of claim 1 wherein the porogen hasa boiling point, sublimation point or decomposition temperature rangingfrom about 150° C. to about 450° C.
 18. The method of claim 1 whereinthe step (c) crosslinking is conducted at a temperature which is lessthan the heating temperature of step (d).
 19. The method of claim 1wherein step (c) comprises heating the film at a temperature rangingfrom about 100° C. to about 250° C., for a time period ranging fromabout 30 seconds to about 10 minutes.
 20. The method of claim 1 whereinstep (d) comprises heating the film at a temperature ranging from about150° C. to about 450° C., for a time period ranging from about 30seconds to about 1 hour.
 21. The method of claim 1 wherein the porogenhas a molecular weight ranging from about 100 to about 50,000 amu. 22.The method of claim 1 wherein the porogen is selected from the groupconsisting of a polyalkylene oxide, a monoether of a polyalkylene oxide,a diether of a polyalkylene oxide, bisether of a polyalkylene oxide, analiphatic polyester, an acrylic polymer, an acetal polymer, apoly(caprolatactone), a poly(valeractone), a poly(methyl methacrylate),a poly (vinylbutyral) and combinations thereof.
 23. The method of claim1 wherein the porogen comprises a polyalkylene oxide monoether whichcomprises a C₁ to about C₆ alkyl chain between oxygen atoms and a C₁ toabout C₆ alkyl ether moiety, and wherein the alkyl chain is substitutedor unsubstituted.
 24. The method of claim 23 wherein the polyalkyleneoxide monoether is a polyethylene glycol monomethyl ether orpolypropylene glycol monobutyl ether.
 25. The method of claim 1 whereinthe porogen is present in the composition in an amount of from about 1to about 50 percent by weight of the composition.
 26. The method ofclaim 1 wherein the composition further comprises a solvent.
 27. Themethod of claim 1 wherein the composition further comprises solvent inan amount ranging from about 10 to about 95 percent by weight of thecomposition.
 28. The method of claim 1 wherein the composition furthercomprises a solvent having a boiling point ranging from about 50 toabout 250° C.
 29. The method of claim 1 wherein the composition furthercomprises a solvent selected from the group consisting of hydrocarbons,esters, ethers, ketones, alcohols, amides and combinations thereof. 30.The method of claim 26 wherein the solvent is selected from the groupconsisting of di-n-butyl ether, anisole, acetone, 3-pentanone,2-heptanone, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyllactate, ethanol, 2-propanol, dimethyl acetamide, propylene glycolmethyl ether acetate, and combinations thereof.
 31. A nanoporousdielectric film produced on a substrate by the method of claim
 1. 32. Asemiconductor device comprising a nanoporous dielectric film of claim31.
 33. The semiconductor device of claim 32 that is an integratedcircuit.
 34. A composition comprising silicon containing pre-polymer, aporogen, and a catalyst selected from the group consisting of oniumcompounds and nucleophiles.
 35. The composition of claim 34 wherein saidcatalyst is metal-ion-free.
 36. The composition of claim 34 additionallycomprising solvent.
 37. The composition of claim 35 wherein saidmetal-ion-free catalyst is tetramethylammonium acetate.
 38. Thecomposition of claim 34 wherein said silicon containing pre-polymercomprises a combination of acetoxy-based leaving groups.
 39. Thecomposition of claim 38 wherein said combination of acetoxy-basedleaving groups comprises tetraacetoxysilane and methyltriacetoxysilane.40. The composition of claim 34 wherein said porogen comprisespolyethylene glycol monomethylether.
 41. The composition of claim 34wherein said porogen comprises polypropylene glycol dimethylether. 42.The composition of claim 34 wherein said porogen comprises polyethyleneglycol dimethylether.
 43. The composition of claim 34 wherein saidporogen comprises polypropylene glycol monobutyl ether.
 44. A precursorfor stable nanoporous film formation comprising said composition ofclaim
 35. 45. A spin-on composition comprising said composition of claim35.
 46. A film comprising said spin-on composition of claim
 45. 47. Amethod of lowering the temperature at which a porous silica film formscomprising the step of adding onium ions or nucleophiles to asilicon-containing prepolymer and porogen.